a(n) = T(n,n-1), where T is array defined in A025564.

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =26a(5) =70a(6) =192a(7) =534a(8) =1500a(9) =4246a(10) =12092a(11) =34606a(12) =99442a(13) =286730a(14) =829168a(15) =2403834a(16) =6984234a(17) =20331558a(18) =59287740a(19) =173149662a(20) =506376222a(21) =1482730098a(22) =4346486256a(23) =12754363650a(24) =37461564504a(25) =110125172682a(26) =323990062452a(27) =953883382354